I have a Phenom II X4 965 and an 890GX mainboard with HT 3.0. I've never OC'd before and I was messing around with the HT link speed and found that it seems to be capped at 2200Mhz.

I have the NB and HT link both set to 2600, but it still won't go past 2200Mhz. I'm pretty sure I'm missing something simple, but despite my Googleing effort I can't seem to find a solution besides raising the FSB.

My bios lead me to believe that I may be messing up some special ratio.

Well...
There is really not much benefit messing with the HT Link speed, it is not like the old FSB where it bottlenecks the system.
The CPU North bridge (IMC) on the other hand is where you really want it max out,
since the L3 on a K10.5 is clocked at NB bridge and it also improves memory performance.

Many boards have stability when OCing the HT Link anyways.
If you really want to OC the HT-Link try adding some voltage on it.

I have the NB and HT link both set to 2600, but it still won't go past 2200Mhz. I'm pretty sure I'm missing something simple, but despite my Googleing effort I can't seem to find a solution besides raising the FSB.

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Overclock the processors bus speed and the HT Link will increase too

You've got a BE, raising the multi might also increase the HT Link but let somebody else confirm that, but rasing the CPU bus will definitely increase the HT link!

Use CPU-Z to see the actual and current speeds in which the HT Link is operating. But as others have said the HT Link doesnt increase performance much, worry about overclocking your CPU's clock speed and memory to the max first.